The use of single-stranded DNA or RNA probes, to test for the presence of particular DNAs or RNAs and associated biological entities in samples of biological material, is well known. See, e.g., Grunstein and Hogness, Proc. Nat'l. Acad. Sci. (US) 72, 3961-3965 (1975); Southern, J. Mol. Biol. 98, 503-505 (1975); Langer et al., Proc. Nat'l. Acad. Sci. (US) 78, 6633-6637 (1981); Falkow and Moseley, U.S. Pat. No. 4,358,535; Ward, et al., European Patent Application Publication No. 0 063 879; Englehardt, et al., European Patent Application Publication No. 0 097 373; Meinkoth and Wahl, Anal. Biochem. 138, 267-284(1984).
Among areas in which such probes find application are testing of food and blood for contamination by pathogenic bacteria and viruses; diagnosis of fungal, bacterial and viral diseases by analysis of feces, blood or other body fluids; diagnosis of genetic disorders, and certain diseases such as cancers associated with a genetic abnormality in a population of cells, by analysis of cells for the absence of a gene or the presence of a defective gene; and karyotyping. See Klausner and Wilson, Biotechnology 1, 471-478 (1983); Englehardt, et al. supra; Ward et al., supra; Falkow and Moseley, supra.
The principle which underlies the use of such probes is that a particular probe, under sufficiently stringent conditions, will, via hydrogen-bonding between complementary base moieties, selectively hybridize to (single-stranded) DNA or RNA which includes a sequence of nucleotides ("target sequence") that is complementary to a nucleotide sequence of the probe ("probing sequence" specific for the target sequence). Thus, if a biological entity (e.g., virus, microorganism, normal chromosome, mammalian chromosome bearing a defective gene) to be tested for has at least one DNA or RNA sequence uniquely associated with it in samples to be tested, the entity can be tested for using a nucleic acid probe.
A DNA or RNA associated with an entity to be tested for and including a target sequence to which a nucleic acid probe hybridizes selectively in a hybridization assay is called "target" DNA or RNA, respectively, of the probe.
A probe typically will have at least 8, and usually at least 12, ribonucleotides or 2'-deoxyribonucleotides in the probing sequence that is complementary to a target sequence in its target DNA or RNA. Outside the probing sequences through which a probe complexes with its target nucleic acid, the probe may have virtually any number and type of bases, as long as the sequences including these additional bases do not cause significant hybridization with nucleic acid other than target nucleic acid under hybridization assay conditions. That is, a probe will be specific for its target DNA or RNA in hybridization assays.
To be useful in analyzing biological samples for the presence of a target DNA or RNA, a polynucleotide probe must include a feature which will render detectable the duplex formed when the probe is hybridized to its complementary sequence in the target (single-stranded) DNA or RNA. Typically, such features in a probe include radioactive atoms or pyrimidine or purine bases chemically modified to include moieties which are readily and sensitively detected by any of a number of techniques.
For example, a probe may be made with .sup.32 P-labelled nucleoside triphosphates; then the probe itself, as well as target DNA or RNA with the probe hybridized to it, can be detected by means of radiation from .sup.32 P-decay.
Probes whose detectability is based on radioactive decay are unsuitable for many applications because of safety problems and licensing requirements associated with radioactive materials and because of degradation of the probes that occurs with radioactive decay during storage. Thus, probes whose detectability is based on chemical modification of pyrimidine or purine bases are preferred in many situations.
There are numerous examples of modified purine or pyrimidine bases in probes wherein a moiety, herein referred to as a "tag moiety," is chemically linked to render detectable target DNA or RNA hybridized with probe. See, e.g. Ward et al., supra; Englehardt et al., supra; Klausner and Wilson, supra. Typically, the "tag moiety" is a moiety to which a protein will bind with high affinity, e.g. an antigen to which an antibody binds; a biotinyl or iminobiotinyl moiety to which avidin or streptavidin will bind; an inhibitor of an enzyme to which the enzyme binds. A protein which binds with high affinity to a tag moiety of a probe is referred to herein as a "conjugate protein" of the tag moiety.
In a typical assay, after probe is hybridized to target DNA or RNA, a "reporter group" is added to the system and binds to the tag moiety or moieties of the hybridized probe. A "reporter group" provides a signal which renders detectable the probe that is hybridized to target DNA or RNA. A typical reporter group is a conjugate protein of the tag moiety or a complex, involving such a conjugate protein, which binds to tag through the binding site for tag in the conjugate protein. The reporter group so bound is then detected by an appropriate immunological, physical, or biochemical technique. For example, if the reporter group is simply a conjugate protein, detection might be by any of a number of well known immunoassay techniques, based on antibodies directed against the conjugate protein. If the reporter group is a conjugate protein which naturally contains a chromophore or fluorophore, or is a conjugate protein modified to include such a moiety, detection might be by a spectroscopic technique based on the chromophore or fluorophore. If the reporter group is a heteropolymer or homopolymer of enzymes, including a conjugate protein, detection could involve detection of substances produced by enzymatic reactions catalyzed by enzymes in the polymer. Ward et al., supra, Englehardt, et al., supra, and Klausner and Wilson, supra, describe a number of techniques for assaying reporter groups bound to tag moieties of probes.
A tag moiety itself, without being bound by a reporter group, might provide detectability to a probe. For example, a tag moiety which is a fluorophore or chromophore can be detected with a suitable spectroscopic technique without binding of a reporter group. See, e.g., Bauman et al., J. Histochem. Cytochem. 29, 227-237(1981).
In some cases wherein pyrimidine or purine bases are chemically modified by the addition of a tag moiety, a linking moiety will separate the tag moiety from the site of modification on the pyrimidine or purine base. See, e.g., the Ward et al. and Englehardt et al. references, supra. In some cases, such linking moieties facilitate the attachment of tag moieties to probe. Further, a linking moiety tends to hold a tag moiety some distance from the modified purine or pyrimidine base, thereby increasing accessibility of the tag moiety to binding by a reporter group and, further, reducing interference with formation or stability of duplexes between probe and target DNA or RNA in those instances where the tag moiety has a large molecular weight.
Polynucleotide probes which comprise at least one cytosine moiety modified to have a tag moiety linked, directly or through a linking moiety, to the N.sup.4 -position, have not been available heretofore.
Because the amino group at the 4-position of cytosine is involved in hydrogen-bonding between cytosine and guanine moieties in nucleic acid duplexes, it has been thought heretofore that modifications to this amino group would be unacceptable in nucleic acid probes. It has been thought that such modifications in a nucleic acid would interfere with duplex formation, and thereby result in a probe with unacceptable specificity and sensitivity, by severely disrupting guanosine-cytosine hydrogen-bonding. See Ward et al., supra; Ruth, Patent Cooperation Treaty International Publication No. W084/03285(1984).
The chemistry of modifying cytosine moieties at the N.sup.4 -nitrogen has been studied with cytidine and 2'-deoxycytidine and their phosphates, both as monomers and included in single-stranded polynucleotides. Nitta et al., FEBS Letters 166, 194-198 (1984); Negishi et al. (I), Nucl. Acids Res. Symp. Series I2, pp. 29-30(1983); Negishi et al., (II), Nucl. Acids Res. 11, 5223-5333 (1983); Hayatsu, Prog. Nucleic Acid Res. and Mol. Biol 16, 75-124 (1976).
The N.sup.4 -amino group, in N.sup.4 -aminocytidine and N.sup.4 -amino-2'-deoxycytidine and their phosphates, both as monomers and included in single-stranded polynucleotides, is known to have reactivities characteristic of substituted hydrazines and reacts accordingly with aldehydes, ketones, isothiocyanates and imidates. Nitta et al., supra; Negishi (I), supra; Hayatsu, supra. See also P. Smith, The Chemistry of Open-Chain Organic Nitrogen Compounds, W. A. Benjamin Inc., New York, New York, Vol. II, pp. 119-209 (1966).
Nitta et al., supra, have reported transamination of cytosine moieties in polycytidine with hydrazine in the presence of bisulfite; and derivatization of the transaminated product with an adduct of glutathione with pyruvic acid, wherein the adduct reacts through the keto-carbon of pyruvate with the N.sup.4 -amino group.